Explain the concept of corners and modes in static timing analysis.

Corners represent PVT (Process, Voltage, Temperature) variations: SS (slow-slow, worst setup), FF (fast-fast, worst hold), typical, and various combinations. OCV (on-chip variation) adds local variations. Modes represent functional configurations: different clock frequencies, power states, or test modes. MMMC (Multi-Mode Multi-Corner) analysis runs STA across all relevant mode-corner combinations. Typically: setup checked at slow corners (worst-case delays), hold checked at fast corners (minimum delays). Signoff may require passing 10-30+ scenarios covering operating conditions.